1. Field of the Invention
The present invention relates to a pressure sensitive circuit breaker (hereinafter referred to as "PSCB") to be used for ensuring the safety of a storage battery such as a secondary battery capable of being regenerated by re-charging. Particularly, the invention is concerned with a PSCB having a rectangular lid.
2. Description of the Related Art
If a storage battery (a secondary battery) of this type is continued to be charged even after completion of charging or if it is charged with an electric current larger than a predetermined current, there may arise a problem such that some trouble occurs in the battery, with evolution of gas therein, increase of pressure and temperature in the battery, swelling of the battery, oozing of electrolyte caused by cracking, and adverse effect on devices installed in the battery. Even if the result is not to such an extent, if the battery is continued to be used in such a state, swelling of the battery will proceed and may cause cracking or burst. Thus, as to a storage battery which has undergone some trouble, it is necessary to stop the use thereof immediately.
Among the devices for which the reduction of size has been required recently, there are many devices which are characterized by their small wall thickness. With this tendency, square or rectangular batteries have come to be used. Also in this case, reduction of thickness remains to be solved. On the other hand, in reply to a keen demand for safety, it is necessary to incorporate a safety valve and a current cut-off mechanism in such devices.
In a conventional PSCB referred to above, when the internal pressure of a storage battery increases due to some abnormal chemical change of a power generating element, the increased pressure acts on a cut-off diaphragm, so that the diaphragm is lifted and an electric conduction passage which connects a central projection and a lead through a lead stripper plate is stripped or broken. As the pressure further increases and reaches a predetermined level, a thin-walled portion of the safety valve is broken and a gas present in a battery case and having the increased pressure is released to the exterior through a hole formed in the lid.
In many of circular batteries, a single member is used for both diaphragm and safety valve.
In such a secondary battery, members incorporating the above mechanisms are laminated together and a junction and a conductive portion for connection between the interior and the exterior are rendered integral and sealed hermetically by caulking with use of a rivet for example.
According to the prior art described above, rivet caulking or fusion-bonding with laser or ultrasonic wave has been used for making integral and seal hermetically such a member as a lid. In this case, however, a strong force is exerted on a flat head of the rivet, resulting in that the head surface of the rivet, which is brought into pressure contact with the lid or a gasket, is flawed or crushed and hence deteriorates the sealability.
Another problem has been encountered such that the member caulked by the rivet creeps due to a change with the lapse of time and thus the sealability and a long-term stability of an electrically contacted state are poor.
Moreover, aluminum material is usually employed on the positive electrode side, so if the rivet used is also made of aluminum, it follows that the aluminum rivet and an internal terminal made of aluminum are connected together and hence the connected state becomes unstable. More particularly, since the gasket is formed of a synthetic resin such as PP, the rivet caulked portion is apt to become loose with the lapse of time, so if the connection is made by rivet caulking, there occurs a gap between the rivet and the internal terminal. As a result, not only the connected state becomes unstable, but also an aluminum oxide film is formed on the interface, resulting in increase of the resistance value and giving rise to a more stable condition.
If welding is adopted for fixation, a damage is done thermally to the caulking portion and it is necessary to ensure a certain degree of thickness. Besides, it is difficult to ensure a receptacle portion for the bonding, and deformation is apt to occur.